It is known, in a satellite communications, for a user terminal, perhaps in the form of a radio telephone handset, not dissimilar to a cellular radio telephone handset, to communicate with an earth station, via a satellite, to establish a telephone call or to receive a telephone call, by means of the earth station linking into the terrestrial wire and cable system or into another radio telephone system.
There are twin purposes relating to establishing the exact position of the user terminal on the surface of the earth. Firstly, in order to know how to direct a radio signal to or from a user terminal, when required, from particular satellite at a particular time, it is necessary to know the approximate location of the user terminal so that the appropriate beam from the appropriate satellite can be selected to cover the portion of the Earth's surface where the user terminal is located. Secondly, in a satellite communication system, in order that call barring, local billing or other restrictions based on the territory wherein the user terminal may be operated can be observed, it is necessary to determine the location of the user terminal with sufficient accuracy for the necessary restrictions to be imposed.
It is known to provide a user terminal where the individual terminal employs "Global Positioning by Satellite" (GPS) to determine, with some great accuracy, the position of the user terminal on the surface of the earth. The user terminal then transmits, to the earth station, via the satellite or satellites involved in communications, its exact position which is then used by the earth station, in subsequent interactions with the user terminal, to control the fiscal and mechanical aspects of the communication activity with the user terminal. An example of such a system is to be found in European Patent EP 0562 374 by Motorola Corporation filed 27th March 1993.
Such systems require multiple frequency capability from the handset, together with enhanced complication of the handset, in order that the handset may be capable both of communications and of GPS measurements.
The present invention seeks to provide a solution to the problem of determining the location of a user terminal or handset, on the surface of the earth, when interacting with a satellite communication system, without the necessity to resort to a solution involving GPS and with sufficient accuracy for the operational and fiscal requirements of a satellite communication system.
A GPS system requires very accurate measured characterisation of the orbits of each satellite so that the position of each satellite can be made known, to a terminal, on the surface of the earth. The terminal itself then calculates its own position. Because the GPS system is one where the satellites alone transmit, information flow is one-way. Measurement of satellite position, which is directly related to the accuracy with which the position of the terminal can be found, is achieved only intermittently by a dedicated facility, and drifting can occur between measurements. The dedicated facility may not be optimally placed on the surface of the earth when the position of the satellite is measured, introducing unwanted errors.
The present invention seeks to provide a continuous updating of the satellite position by exploiting the two-way characteristics of a satellite communications system.